What is an uncraftable potion?

Regarding the uncraftable potion -

The CustomPotionEffects is a list, so everything in it has to be put into square brackets ([]).

And for some reason custom potion effects, as the only place in the game, require "Id" with a capital "I/i". Note that the non-capital "id" is indeed correct for the enchantments.

Putting these two corrections together, the correct command should be:

  give @s minecraft:splash_potion{CustomPotionEffects:[{Id:7,Amplifier:3,ShowParticles:0}],Enchantments:[{id:"minecraft:multishot",lvl:1}],display:{Name:"{"text":"Holy Hand Grenade of Antioch","colour":"gold"}"}} 2

Source: I always start at "chunk format" as the central hub for everything, because I'm lazy, which directs me to "Player.dat#Item structure", specifically to Potion effects (archive).